1. Field of the Invention
The present invention relates to a control system for electromagnetically controlling the operation of a valve which opens and closes an intake/exhaust port through which the interior and exterior of an engine cylinder are connected.
2. Description of the Related Art
Conventional control systems for controlling the opening and closing intake and exhaust valves operate as follows. When fuel is burned in a combustion chamber, the piston is lowered to cause the connecting rod to rotate the crankshaft. A camshaft disposed in the cylinder head is driven by the crankshaft and a timing belt, and the intake and exhaust valves are opened and closed by rocker arms held against cam surfaces of the camshaft.
Since the timing with which the intake and exhaust valves are opened and closed cannot be altered during operation of the engine, the valve opening and closing timing is preset such that the engine operates with high efficiency when it rotates at a predetermined speed.
More specifically, the timing with which the intake valve is opened is selected as a crankshaft angle ranging from 20.degree. to 30.degree. before the top dead center (TDC) of the piston is reached, and the timing with which it is closed is selected as a crankshaft angle ranging from 50.degree. to 60.degree. after the bottom dead center (BDC) is passed.
The timing with which the exhaust valve is opened is selected as a crankshaft angle ranging from 50.degree. to 60.degree. before the bottom dead center (BDC), and the timing with which it is closed is selected as a crankshaft angle ranging from 20.degree. to 30.degree. after the top dead center (TDC).
With the conventional settings for the timing with which the intake and exhaust valves are opened and closed, the intake valve remains open even after the bottom dead center (BDC) is passed. As a result, when the engine is operating at a speed lower than the above predetermined speed, an air-fuel mixture which has already been drawn into the cylinder is discharged back out of the cylinder through the intake port as the piston moves upwardly, resulting in a reduction in the engine output power.
Similarly, after the top dead center (TDC) is passed, the exhaust valve still remains open for a certain period of time. As a result, an unburned air-fuel mixture that was introduced from the intake port does not stay in the cylinder, but is discharged from the cylinder through the exhaust port, resulting in poor fuel economy.
Valve control systems which use electromagnetic means rather than camshafts for driving intake and exhaust valves are disclosed in Japanese Laid-Open Patent Publications Nos. 58-183805 and 61-76713. However, the disclosed valve control systems are not designed to solve the above problems.